thank uUpthrust is caused by difference in pressure at the top and bottom of the block.
The pressure is due to WATER. Pressure changes with height but since all blocks have the same height and all are in the same fluid thus all blocks will have an equal difference of pressure between the top and bottom. Therefore all of them will experience same Upthrust.
The formula pressure = h × D ×g is for fluids and the density is of the fluid so it's the desnity of liquid that matters NOT the object immersed
This is a simple moments question.
The force L was moved to the right.